1. Field of Invention
At least one embodiment of the invention relates to a system and method for controlling aeration and/or actuation of a surface ventilation propeller of a marine vessel.
2. Discussion of Related Art
A surface-piercing propeller (or surface propeller) is a propeller that is positioned so that when the vessel is underway the waterline passes right through the propeller's hub. This is usually accomplished by extending the propeller shaft out through the transom of the vessel, and locating the propeller some distance aft of the transom in the relatively flat water surface that flows out from the transom's bottom edge. (The exception being single-shaft catamarans, where the propeller hub intersects the undisturbed waterline.) In the case of articulated surface drive systems, the propeller shaft is driven through a double universal joint inside an oil-tight ball joint, allowing the shaft to rotate athwartships for steering and to trim up and down for control of propeller submergence. Fixed-shaft surface drives can use conventional shafts and stern tube bearings, but require rudders. In many racing applications, outboards and outdrives can be positioned sufficiently high on the vessel for the propellers to operate in a surface-piercing mode. The primary operating feature of a ventilating propeller is that each propeller blade is out of the water for approximately half of each revolution.
Traditional propeller design and selection is almost always an exercise in trading off diameter against several other performance-limiting parameters. Basic momentum theory tells us that for a given speed and thrust, the larger the propeller, the higher the efficiency. While there are exceptions, most notably the effects of frictional resistance on large, slow-turning propellers, it is generally borne out in practice that a larger propeller with a sufficiently deep gear ratio will be more efficient than a small one.
A number of design considerations conspire to limit the maximum feasible propeller diameter to something considerably smaller than the optimal size. These include blade tip clearance from the hull, maximum vessel draft, shaft angle, and engine location. While this may at times make life easy for the designer—the propeller diameter specified is simply the maximum that fits—it can also result in a considerable sacrifice of propulsive efficiency. And if these geometric limits on propeller diameter are exceeded, the result can be excessive vibration and damage due to low tip clearances, or a steep shaft angle with severe loss of efficiency and additional parasitic drag, or deep navigational draft that restricts operation or requires a protective keel and its associated drag. In many cases, the best design solution is to live with a mix of all of the above problems to some degree. The surface-piercing propeller frees the designer from these limitations. There is virtually no limit to the size of propeller that will work. The designer is able to use a much deeper reduction ratio, and a larger, lightly-loaded, and more efficient propeller.
When a submerged propeller blade cavitates, the pressure on part of the blade becomes so low that a water vapor cavity is developed. When these water vapor cavities collapse, water impacts on the blade surface with a local pressure singularity—that is, a point with theoretically infinite velocity and pressure. The effect can approximate that of hitting the blade with a hammer on each revolution. Cavitation is a major source of propeller damage, vibration, noise, and loss of performance. And although high-speed propellers are often designed to operate in a fully-cavitating (supercavitating) mode, problems associated with cavitation are frequently a limiting factor in propeller design and selection.
The surface propeller effectively eliminates cavitation by replacing it with ventilation. With each stroke, the propeller blade brings a bubble of air into what would otherwise be the water vapor cavity region. The water ram effect that occurs when a vacuum cavity collapses is suppressed, because the air entrained in the cavity compresses as the cavity shrinks in size. Although the flow over a superventilating propeller blade bears a superficial resemblance to that over a supercavitating blade, most of the vibration, surface erosion, and underwater noise are absent.
Note that cavitation can also be associated with sudden loss of thrust and high propeller slip, often caused by a sharp maneuver or resistance increase. This can still occur with surface propellers, although the propeller is ventilating rather than cavitating and the result is not as damaging.
Exposed shafts, struts, and propeller hubs all contribute to parasitic drag. There is also a considerable amount of power loss resulting from the friction of the shaft rotating in the water flow. In fact, for conventional installations a net performance increase can often be realized by enclosing submerged shafts in non-rotating shrouds, despite the increase in diameter.
Surface propellers virtually eliminate drag from all of these sources, as the only surfaces to contact the water are the propeller blades and a skeg or rudder. When a surface propeller is used in conjunction with an articulated drive system, the vessel operator then has the ability to adjust propeller submergence underway. This has roughly the same effect as varying the diameter of a fully submerged propeller, and allows for considerable tolerance in selecting propellers—or it allows one propeller to match a range of vessel operating conditions. This capability is somewhat analogous to adjusting pitch on a controllable pitch propeller. FIG. 2A-B illustrate an example of an articulating drive system that includes a ventilating propeller 102. The propeller submergence below the free surface is adjusted by actuating the trim cylinder 204 up and down. When an articulated drive is used for steering, the result can be exceptionally good high-speed maneuvering characteristics. On single-shaft applications, drive steering can also be used to compensate for propeller-induced side force, without resorting to an excessively large rudder or skeg.
Most planing hull designs, especially moderately low-powered or heavy designs, are subject to problems getting through “hump” speed. High vessel resistance at pre-planing speeds, high propeller slip, and reduced engine torque output at less than full RPM can sometimes combine to make it impossible to reach design speed, even though the vessel may be perfectly capable of operating at design speed once it gets there. The boat that “can't get out of the hole” is a phenomenon that should be quite familiar to many designers and builders. With surface propulsion systems there is an additional factor which may make the situation worse—the propeller is designed to operate with only half of the blade area immersed below the waterline (illustrated by the planning speed free surface 110 in FIGS. 1, 2A-B & 3A-B). But at low speeds, before the transom aerates or “drys out,” the propeller must operate fully submerged (this condition is illustrated by the propeller below the non-planing speed free surface 108 in FIGS. 1, 2A-B & 3A-B). Not only is the submerged area doubled, but the top half is operating in very strong wake turbulence right behind the transom 112. The result is that it takes much more torque to spin the propeller at a given RPM, and sometimes the engine is not capable of providing the torque necessary to turn the propeller fast enough to get the boat up to the speed to allow the transom to aerate and unload the top half of the propeller.
To reduce this potential problem, various methods of aerating the top half of the propeller have been employed. On some installations, passive (fixed) “aeration pipes” leading from above the static waterline to the forward side of the propeller have been effective. Examples of ventilating propellers that use aeration pipes with various drives systems are shown in FIGS. 1, 2A-B & 3A-B. FIG. 1 illustrates a fixed surface drive including a ventilating propeller 102 with an aeration pipe 104. The system illustrated in FIGS. 2a-b is an articulating surface drive that articulates the ventilating propeller 102 by an articulating drive shaft 208 that is pivoted up and down via actuation of the trim cylinder 204. The aeration pipe 202 in the figure is fixed; however, it could also be fixed to the articulating drive shaft housing and move up and down with the drive. FIGS. 3a-b illustrates a surface propeller 302 that is installed on an out-drive 306. The out drive can be articulated side to side for steering and up and down to adjust the propeller submergence. An aeration pipe 304 is fixed to the lower drive section 308 in order to maintain the proximity of the pipe 304 to the ventilating propeller 302 when the lower drive section 308 is moving.
The above-described systems can be improved upon.